The present invention relates generally to liquid density monitoring apparatus and is particularly concerned with monitoring suspended sediment concentration in water for hydrologic purposes.
Density monitors typically employ a hydrometer bulb floating or suspended in the liquid to be monitored. The elevation of the bulb in the liquid will be a direct function of the density. Various techniques have been used to monitor change in level of a hydrometer bulb in a liquid. For example, in U.S. Pat. No. 3,089,502 of Davidson et al., the bulb has an elongated stem with a magnetic core at its upper end which is vertically slidable relative to the core of a differential transformer, so that the change in current through the transformer will be proportional to the change in level of the hydrometer bulb. A similar arrangement utilizing a differential transformer to monitor bulb level is described in U.S. Pat. No. 3,407,666 of Glassey, where the hydrometer bulb is of stainless steel and/or Teflon.RTM.. U.S. Pat. No. 3,186,423 of Davidson and Glassey describes another density control instrument employing a suspended hydrometer bulb having a magnetic core slidable within the coils of a differential transformer. In this instrument, temperature compensation is provided since the density of a liquid will be a function of its temperature. A coil link supporting the armature or magnetic core is of a material which expands and contracts in response to temperature change, thus changing the length of the coil and the core position as a function of temperature change. The armature is therefore raised or lowered to compensate for temperature variations, making the output essentially independent of temperature changes.
Another technique which has been proposed for measuring liquid density uses a plummet suspended from a mechanical balance into the liquid being monitored, with the effective weight of the plummet being a function of the density. Liquid density is related to the concentration of suspended sediment in a liquid, and it has therefore been proposed that such a technique be used to monitor sediment concentration in bodies of water such as rivers, for example. As a liquid becomes more dense due to increased sediment levels, the buoyant force acting on the plummet increases, decreasing its effective weight. Thus, the variation in the plummet weight from a reference value will be proportional to the bulk density, and thus the sediment concentration. However, one problem with using a standard mechanical balance to measure such effects is that the accuracy is very limited. Additionally, temperature variations will effect the accuracy of the output.